1. Field of the Invention
This invention relates generally to an apparatus for controlling the speed of a moving object, and more particularly to an apparatus for controlling the speed of a control unit, such as a servo motor, in which there must not be any speed change or position change while the control unit is operating. Here in this specification, the term "moving object" means not only an object which moves by itself or varies in speed, but also an object which moves a load or varies in speed in association with a load.
2. Description of the Related Art
Attempts have been made to keep constant the rotating speed of a drive motor for a videotape recorder (hereinafter called VTR). If there is any speed fluctuation (i.e., irregular rotation, speed ripple or torque ripple), an image would be disordered, thus markedly deteriorating the reliability and quality of VTR.
To this end, in many of the conventional apparatuses, a d.c. motor has been used, and recently incorporating a brushless motor, which can change the speed freely and simply, has become commonplace.
A brushless motor has no mechanical brush and can hence eliminate various kinds of problems due to the wear of a brush and a commutator or the metal dust resulting from such wear. However, in a 120.degree.-conducting brushless motor, the number of crossovers of magnetic bundles varies, depending on the position of a rotor, so that a torque ripple would occur, thus causing an irregular rotation (speed fluctuation) during operation.
Now assuming that the number of crossovers of magnetic bundles of a drive-phase coil is K (.theta.) (.theta. is the position of a moving object), a torque K(.theta.)i (i is a current flowing in the coil) will be produced. If the current is constant, a torque ripple will be produced in proportion to K(.theta.).
This torque ripple leads to disturbance, causing a speed fluctuation.
Consequently, the speed fluctuation has a periodicity with respect to the rotational position of the motor. A technique of correcting the speed fluctuation, utilizing this characteristic, has been proposed by Japanese Patent Laid-Open Publication No. 218380/1989. According to this technique, the feature of the speed fluctuation is sampled by Fourier progress expansion, and a proportional and integrating control operation is made in such a manner that this sampled value will be zero, whereupon the current which flows in the coil is varied upon receipt of a signal obtained from Fourier reverse transformation of this operation output, thus correcting the speed fluctuation.
In the technique of the Japanese publication, an expensive arithmetic and operation unit is necessary in which very many multiplications and additions are executed at high speed, when performing Fourier progress expansion and Fourier reverse transformation, thus a lot of time is taken to perform such mathematical operations.
Further, in the method of the Japanese publication, assuming that the average load fluctuation is small, an optimum operation can always be achieved if a correcting value is determined only once. In the absence of a drastic load fluctuation such as in a servo motor, production of a correcting torque cannot catch up with the fluctuation so that a smooth operation cannot be realized.